The invention relates to a transfer apparatus used in copying machines.
A copying machine of the type in which a toner image formed on a photosensitive member is transferred onto a sheet is provided with a transfer charger which applies a corona discharge to the rear surface of the sheet which is of the opposite polarity from that of the toner in order to transfer the toner from the photosensitive member to the sheet and to maintain the sheet in close contact with the member. The transfer of toner and the close contact between the member and the sheet can be improved by disposing the transfer charger as close to the photosensitive member as possible. In a copying machine in which the sheet is conveyed while it is being held by a sheet gripper, there must be provided a sufficient clearance between the photosensitive member and the transfer charger to permit the passage of the sheet gripper, and this prevented the disposition of the transfer charger sufficiently close to the photosensitive member. As a consequence, a copying machine of this type suffered from a poor transfer quality of the toner image.
It is believed that the bonding force between the toner and the photosensitive member depends on the particle size, charge and mass of the individual toner particles, and hence varies from toner particle to toner particle. This results in a movement of the toner having a reduced bonding force with respect to the photosensitive member to a position on the sheet which is offset from an accurate position before the sheet is brought into contact with the photosensitive member because of the application of the corona discharge of the opposite polarity, from that of the toner, by the transfer charger. Consequently, the optical density will be reduced where an image is to be transferred, while the toner which initiated its movement before the intended transfer takes place will cause a background smearing of the sheet. Such difficulty can be avoided by providing a reduced opening in a shield casing of the transfer charger so that the corona discharge from the electrode thereof will be applied only to a region of the photosensitive member and the sheet which are in contact with each other. However, with conventional copying machines which employ a sheet gripper, the spacing required between the photosensitive member and the transfer charger to permit the passage of the gripper therethrough prevented the disposition of the transfer charger close to the photosensitive member as well as the provision of a physical diaphragm member which serves reducing the opening in the shield casing.